A spectrometer is a device capable of separating an input light source into its constituent spectral components and separately measuring the intensity of each such component. Spectrometers can be further classified by the nature of their dispersive element, which can be a prism or diffraction grating. In addition, if the entire dispersed spectrum is measured simultaneously by means of a photographic plate or detector array it is described as a spectrograph, and if each spectral component is presented separately to a single detector it is commonly known as a monochromator.
The following disclosure presents a number of unique features which, when incorporated into the design of a grating spectrograph, greatly facilitates performance and ease of manufacture. An appreciation of the advantages these features represent when compared with previous designs can be derived from consideration of the basic Czerny-Turner layout of a grating spectrometer as shown in FIG. 1.
The light which is to be analyzed is presented to the spectrometer through an entrance slit (1). To control the divergence of this source inside the spectrometer and ensure it does not lead to overfilling of the entrance minor, M1 (3), an aperture (2) is used after the slit to limit the acceptance cone to only light that will strike the input mirror (3). The aperture (2) size is usually expressed as an f-number by comparing the size of the beam at M1 (3) to the focal length of M1 (3).
Mirrors are used as focusing elements in order to avoid the problem of chromatic aberration, which is present to some extent in any system employing lenses.
Because lenses cannot be employed M1 (3) is a curved mirror. In order that the light reflected from it be directed towards the diffraction grating (5), M1 (3) is necessarily used as an off-axis collimating element. This introduces a variety of aberrations into the collimated beam due to astigmatism and coma. The result is that the nominally collimated beam actually contains a distribution of angles.
The diffraction angle from the grating (5) depends non-linearly on the angle of incidence, so the angular distribution present in the incident beam is broadened in the diffracted beam. Furthermore, since the diffraction angle is also a function of wavelength, the output distributions differ for the various wavelength components of the beam.
M2 (4) is also used as an off-axis element, and therefore contributes its own aberrations into the image formed at the detector (6). M2 (4) must be larger than M1 (3) in order to avoid vignetting the dispersed light diffracted from the grating (5). Since different portions of M2 (4) are used by different wavelength components, the aberration contributions are different as well.
Finally, note that for any given layout of the mirrors (3 & 4) and grating (5) only a single spectral component will encounter the center of each of these elements. Due to diffraction, other spectral components will diverge from the path of the center component to a greater or lesser extent depending on their wavelength and angle of incidence at the grating (5). If their divergence is too large they will miss either the second mirror (4) or the sensitive area of the detector (6). Thus, any given configuration will have a wavelength range, characterized by a center wavelength and a minimum and maximum wavelength.
These issues are all well-known, and are traditionally addressed by designing the optical system to minimize both the fold angles and thus, necessarily, the input aperture. By holding both parameters to the smallest possible values the aberrations are minimized.